The present invention relates to an improved, low energy fuse for use in commercial blasting, improved materials useful in its manufacture and to a method for producing such a fuse.
The use of non-electric explosives initiation systems is now well known in the blasting art. Generally, these systems comprise the use of one or more lengths of detonating fuse cord each having attached at one end thereof an instantaneous or delay blasting cap. When the opposite end of the cord is initiated by means of an explosive initiator, such as a cap or priming trunk line fuse cord, the detonating fuse is detonated and an explosive wave is transmitted along its length at high velocity to set off the attached blasting cap. The use of such a system is generally chosen where there may be hazards involved in using an electric initiation system and electric blasting caps.
In the past, many improvements have been made in the quality and reliability of non-electric initiation systems and in detonating fuse cord. An early but significant development was disclosed in our British patent No 808 087 (equals U.S. Pat. No. 2,993,236). This provided a solution to the problem of how to safely incorporate an explosive core in a thermoplastic tubular sheath during extrusion. The technique disclosed therein can be widely applied to production of tubular products for use in initiation systems. One such product is shown in British Patent No. 1 238 503 (equals U.S. Pat. No. 3,590,739; CA 878 056) which discloses a detonating fuse which comprises a tube having only a thin layer of a reactive substance coated on the inner area thereof rather than a core. Such a fuse is marketed under the registered trade mark xe2x80x9cNONELxe2x80x9d. Commonly, this type of fuse has come to be known as a shock wave conductor and will be referred to as such hereinafter.
The production of shock wave conductors of small diameter has been restricted to use of a limited number of polymers due to the principal properties sought for the product. The product development trend in the art to meet such problems has been to provide laminated plastics tubes comprising an inner and outer layer of differing plastics to satisfy requirements of reactive substance adhesion and mechanical strength respectively. A shock wave conductor in the form of a two-ply laminated tube, the outer ply of which provides reinforcement and resists mechanical damage, is disclosed in GB 2 027 176 (U.S. Pat. No. 4,328,753; CA 1 149 229). Likewise in U.S. Pat. No. 4,607,573, a method is described for the manufacture of a two-ply or multiply shock tube wherein the outer covering is applied only after the inner tube has been stretched to provide the desired core load per unit length. Further examples of such over coated tubes are disclosed in U.S. Pat. No. 4,757,764 which proposes use of the tubes of the type disclosed in the above-mentioned U.S. Pat. No. 4,607,573 with non-self-explosive reactive material within the tube. Other disclosures of the use of non-self-explosive reactive material are to be found in Brazilian Patent No. PI 8104552, CA 878 056, GB 2 152 643 and U.S. Pat. Nos. 4 660 474 and 4 756 250.
While the invention of the shock wave conductor has been an important contribution to the art of blasting, the known shock wave conductors are not without disadvantages. Since the reactive substance within the tube only comprises a thin surface coating which adheres to, but is not bound to the tube, then only certain special plastics have in practice been found suitable to provide the necessary adhesion. Such special plastics tend to be both expensive and to lack mechanical strength. When protected by an outer layer of material, as disclosed in U.S. Pat. Nos. 4,328,753 and 4,607,573, the mechanical properties are improved.
A need has arisen, therefore, for a shock wave conductor which retains all the explosive properties of the tubes currently in use and which is also possessed of great mechanical and tensile strength but at low production cost.
According to the present invention, a low energy shock wave conductor is provided which comprises an extruded single-wall, dimensionally stable plastic tube having an inner surface coated with a particulate reactive energetic material, the plastic of the said tube comprising a substantially homogeneous blend of a major amount of a draw orientable polymer resin lacking adequate reactive material-retaining properties, and a minor amount of a modifier which is a miscible or compatible material which imparts an enhanced reactive material-retaining capability to the said extruded plastic tube.
Most favourable results are achieved in most instances when the polymer is substantially orientated linearly and this is best achieved by cold drawing the tube after melt consolidation. As used herein the term xe2x80x9ccold drawingxe2x80x9d means irreversible extension with a localised draw point of the extruded tube at any stage after the polymer has left the extruder and cooled sufficiently to consolidate a permanent tubular structure but remains plastic or sufficiently so to permit stretching under applied stress to thereby orientate the crystallites in the direction of tube length. Thus cold drawing may be carried out at any stage after the tube has taken shape after extrusion and has begun to cool from its extrusion temperature. Therefore it should be noted that the temperature of xe2x80x9ccold drawingxe2x80x9d lies suitably in the range of from about ambient room temperature to about 180xc2x0 C. or higher depending on the polymer(s) chosen and it will be recognised that the temperature profile of the cold drawing stage(s) need not be uniform so that the post-extrusion temperature treatment of the tube may be variable. Additionally, intermediate or terminal relaxation stages may be employed, as are well known in the synthetic fibre art, to xe2x80x9cstress relievexe2x80x9d the cold drawn tube and thereby impart improved dimensional stability to the tube. It is envisaged that normally artificial cooling of the extruded tube will be applied such as forced air and/or water cooling to control the temperature during post extrusion treatment. The resulting tube is safe to handle and is easily reeled for storage or transport. Of course the finished tube may be treated externally with agents to improve resistance to water and oil, especially diesel, permeability. Ordinarily a thin film or coating will suffice. Alternatively, the polymer blend may include a further resin to improve oil resistance. The tube can be overcoated with another layer of polymer as in the prior art tubes but there is no perceived advantage in doing so.
Tests, including microscopic examination, carried out on the improved tubes made so far in accordance with the invention indicate that the draw-orientable polymer resin is in the form of a continuous matrix whilst said compatible material is mostly present within the matrix as discrete noncontiguous particles, sized about 0.5 xcexc, or fibrils a few microns in length, with aspect ratios typically of from about 6 up to about 10 oriented along the tube axis. The structural state of said miscible material is less certain because inherently there are no clear phase boundaries to be highlighted by electron microscopy. However we have noted that those miscible polymeric materials that impart good particle adhesion properties at the inner tube surface appear to be present to a substantial extent as indistinctly segregated zones of more concentrated material. Thus electron microscopy (viewing regions up to 20xcexc across) reveals arbitrary random microstructure in the plastic matrix consistent with such zoning. It has further been observed that in many instances the miscible or compatible material is, following melt extrusion, distributed such that it has a greater concentration at the inner surface of the tube than in the body of the matrix which provides optimum exposure to interaction with the reactive material and favorable performance in the resulting shock wave conductor. The distribution of the miscible or compatible material will vary depending on the physical and chemical properties of the selected material.
The polymer tube components may be pro-blended in a suitable mixer prior to supply to the melt extrusion equipment to ensure proper mixing of material with the matrix polymer. The observed surface enrichment upon melt extrusion is a surprising effect and provides a surface presence of the desired powder adherent material substantially larger than the population of components in the tube material would imply. This phenomenon is believed to be achievable by a number of mechanisms, or a helpful combination of such mechanisms, depending on the particular polymer matrix and powder adherent materials present. Presently favoured explanations are first preferential wetting or coating of the extrusion die surfaces by the dispersed material in the molten polymer matrix, and second migration of material under shear gradients in the extrusion head to the die head surface, i.e. rheological causes. The evidence of inner surface enrichment both in the as-extruded tube and that following cold drawing is scientifically demonstrable by use of well known physical techniques such as ESCA.
The miscible or compatible material is preferably a miscible or compatible polymer or copolymer resin or a lower molecular weight material of like properties capable of improving reactive material-retaining properties of the matrix polymer by one or more of the following mechanisms; (i) chemical interaction such as ionic or hydrogen bonding; (ii) physical interaction such as polar attraction, tack or surface-wetting and (iii) electrostatic interaction with the selected reactive material. In fact virtually any material which can be successfully introduced to the bulk matrix-forming polymer and survive the extrusion process without degenerating or disrupting the formation of the tube can be used provided it has the capability to impart the desired improvement in reactive material-retaining property to the matrix polymer. Suitable materials can be recognised by their compatibility with the selected bulk resin and by having pendant or free functional groups which will interact with the chosen reactive material by e.g. polar attraction, hydrogen bonding, ionic attraction without necessarily forming an ionic bond. Alternatively the molecular structure is such that interaction is by physical attributes such as tack, high surface energy or surface conditions e.g. roughness which could be modified by inclusion of ultrafine fillers such as silica at levels of perhaps 0.5-1.0%.
The bulk polymer matrix of which the tube is mainly composed broadly comprises olefinic polymers, including ethylene/alpha-olefin copolymers where the olefin monomer may have from 4 to 16 carbon atoms such as 1-butene, 1-hexene, 1-octene, 4-methyl-1-pentene etc. These typically have a melt flow index of from 0.1 to 2 and a density of from 900 to 950 kg.mxe2x88x923. In general suitable matrix polymers will be fibre forming polymers. Advantages of these polymers are their ease of processing in extrusion equipment, structural strength and generally lower cost that current shock tube components.
The plastic preferably also comprises a minor amount of a polymer or copolymer resin or cross-linking agent which is miscible in the said matrix polymer resin and which imparts melt strength and aids in tube extrusion. Such a material may be an ethylene/acrylic acid ester copolymer or a copolymer of ethylene and vinyl acetate. The acrylic esters are preferably lower alkyl esters such as methyl or butyl acrylates.
Thus a suitable tube comprises a blend of 60 to 97% by weight of a polyolefin resin, e.g. linear low density polyethylene, (optionally including from 5 to 45% weight of a second resin which is a polyolefin-miscible or compatible polymer, copolymer or cross-linking agent which imparts melt strength to the blend and aids in tube extrusion) and from 2 to 25%, preferably up to 10%, by weight of a third polyolefin-miscible or compatible resin which is a surface property modifying polymer or copolymer such as an ethylene/acrylic acid or methylacrylic acid copolymer which may be wholly or partially neutralized e.g. an ionomer such as Surlyn 1855 (Trade Mark for a Du Pont product).
A linear low density polyethylene which may constitute up to about 97% of the polymer blend and which is used in a preferred embodiment of the tube of the invention desirably has a melt flow index (MFI) of around 1.0. The polyethylene-miscible or compatible resin which imparts melt strength to the polymer blend can advantageously be, for example, ethylene/vinyl acetate copolymer or a low density polyethylene having a melt index of 3 or less. The polyethylene-miscible or compatible powder-retention enhancing resin may be any acidic or ionomeric-based co-polymer such as, for example, PRIMACOR, an ethylene-acrylic acid copolymer, sold by Dow Chemical Company.
The method of the invention comprises the steps of extruding a melt of the blended constituents of the plastic tube through a wide annular die in the form of a thick walled tube while distributing particulate reactive energetic material in a core load per unit length on the inner wall of said thick walled tube and elongating the said thick walled tube to form a localized drawing point by cold drawing, to increase the tube tensile strength, to reduce the said wall thickness and to reduce the core load per unit length of the said reactive material. The manner of extruding the thick walled tube whilst introducing the core load of reactive material is similar to that disclosed in GB 808 087 (U.S. Pat. No. 2,993,236) and is widely understood by those in this art. The sizes for shock tube are virtually standardized throughout the art at approximately 3 mm O.D. and 1 mm I.D. by the need for compatibility with existing detonators etc. Thus it will be apparent to those skilled in the art that sizing dies, where required, amount of melt drawing and cold drawing will be selected to provide an equivalent or different sized product. It may be suitable to start from extrusion of a primary tube of about 6 to 10 mm O.D. and about 3 mm I.D. Significant drawing below tube consolidation temperatures may be most appropriate. However in view of the diversity of compositions now discovered to be useful for producing such tubes it is not considered that definite ranges can be specified for drawing. However a natural draw ratio of at least 4:1, weight for weight of equal lengths of undrawn against drawn tube, may be most favorable which is perhaps equivalent to a mechanical draw ratio of about 5 to 8:1 Therefore, due consideration must be had to the type of matrix polymer chosen and any necessary minor operating adjustments ascertained by brief preliminary trial or experimentation. Guidelines for same may be determined from the non-limitative Examples hereinafter given.
The plastic tube shock wave conductor is preferably manufactured in such a manner as to provide a tensile strength of up to 170 newtons per square millimetre. An effective minimum coreload for high velocity shock tubes would be about 15 mg.mxe2x88x921 but loadings of reactive material of up to 20 mg.mxe2x88x921 are possible, or even higher as indicated in the above-mentioned specifications e.g. 25 to 60 mg per linear meter as indicated in U.S. Pat. No. 4,757,764. Tube dimensions are a matter of choice and would be affected by the required internal diameter and the need to obtain a self-supporting tube but normally these would be from 2.5 to 3.3 mm O.D. and about 1.3 mm I.D.
Suitable materials for use as the draw orientable matrix polymer include linear polyethylenes such as those currently commercially available under the Trade Marks xe2x80x9cAecithenexe2x80x9d, particularly LF 3020, LF 3081 and LF 3100; xe2x80x9cDowellexxe2x80x9d, especially 2045-A, 2049 and 2075; Du Pont 12J1; Esso 3121.73; Idemitsu polyethylene-L 0134H; Mitsubishi polyethylene-LL H20E, F30F and F30H; Mitsui xe2x80x9cUltzexxe2x80x9d 2020L, 3010F and 3021F; Nippon NUCG-5651 and Union Carbide DFDA-7540, which are all believed to be essentially LLDPE""s, but equally MDPE, HDPE, ULDPE and LDPE can be used to form plastic tubes in a satisfactory manner. Blends of these polyolefins are also considered useful, especially LLDPE with HDPE due to their close compatibility which is believed to arise from cocrystallisation. Ethylene/propylene copolymers such as EXXELOR(trademark) PE 808 (Exxon Chemicals Ltd.) and polypropylenes such as PROPATHENE(trademark) (ICI) are also useful for the present purpose. Likewise, copolymers of these polyolefins with substituted olefins is possible.
Due to variations in commercially available bulk polymers some initial experimentation and minor variation of the extrusion process may be required but such is believed to be within the ordinary skill of those in the art. Apart from the above olefinic polymers which are favored in terms of availability, cost, processability and physical properties, when extruded to form a shock tube, other draw-orientable melt-extrudable polymers of sufficient toughness and possessing adequate water and oil resistance may be used e.g. polyesters such a polyethylene/butyleneterephthalate (PBT) of nylons may also be used as a basis for the structural polymer matrix of the tube with similar results. Kodar(trademark) is a suitable polyester obtainable from Eastman Chemicals. The diversity of polymers available in the plastics extrusion-moulding field and synthetic fibre field is now so vast that it is impossible to test them all but the expertise available in those fields will permit an informed exploration of other polymers should that be desired.
The polymer that provides the bulk matrix of the tube is simply required to provide a tough tube of the desired dimensions and physical properties and to be an adequate carrier for the incorporated material that serves to impart powder adherent/retentive properties to the inner tube surface. It needs, of course, to be melt extrudable in a manner allowing effective powder introduction and therefore to possess, or be given, adequate melt strength. Many of the preferred bulk polymers, e.g. LLDPEs, are melt-thinning under shear and therefore require either highly skilled extrusion expertise or, if a more forgiving polymer melt is desired, a sufficient but small proportion of melt blended miscible melt strength additive as described further below.
The basic and surprising discovery from which the present invention is derived is that for a practical shock wave conductor tube a bulk powder adherent homopolymer is not needed contrary to the long standing belief and practice of the art. A blend in which there is separation of function can work as well or better and be economically advantageous.
The particulate reactive material required for sustaining a shock wave within the tube requires the surface presence of an additive which according to the present invention may be in the form of another polymer, or a lower molecular weight material, which is sufficiently miscible or compatible as to be incorporated in the bulk polymer matrix to provide an extruded tube exhibiting the desired retentive properties. The additive must not be excessively binding nor exhibit aggressive tack or rely solely on transient electrostatic properties since the reactive material would then be incapable of propagating the shock wave either by being permanently attached to the tube surface or through migration from the surface over a period of storage. Thus we have found that selected materials should be added to the matrix polymer prior to extrusion to provide an extrudable blend capable of being drawn to form a satisfactory tube for use as a shock wave conductor. These are characterized by having pendant or free functional or polar groups e.g. carboxyl, anhydride, hydroxyl, halogen, cyano, amido, sulphonate etc., by having an inherent adherent property or by being of relatively small molecular size. Such materials can be selected from ethylene/acrylic acid (EAA) copolymers, ethylene/methacrylic acid (EMA) copolymers, polyisobutylenes (PIB), polybutadienes (PBD), polyethylene waxes (PE Wax), ionomers, polyethylene glycols (PEG), poly-propylene glycols (PPG), ethylene vinyl alcohol resins (EVAL), buryl rubber, Rosin, maleinised polypropylene, polyacrylamide or polyacryl-amide oxime resins, polyethylene imine, sulphone or phosphonate resins. Preferably the additive is an ethylene acrylic acid copolymer (EAA) or methacrylic acid copolymer (EMA), or an ionomer. Polymers suitable for this purpose include those commercially available under the Trade Marks xe2x80x9cPrimacorxe2x80x9d (EAA), e.g. 1430, xe2x80x9cSurlynxe2x80x9d 1855 (believed to be wholly or partially neutralized polymers of methyl acrylic acid and ethylene monomer) or 8940 (Na ionomer), xe2x80x9cNucrelxe2x80x9d (EMA) 403 or 410, Hyvis 30 (PIB, BP Chemicals), Lithene N4 6000 (PBD, Doverstrand Ltd), Soarnol D (EVAL resin, British Trades and Shippers), Portugese WW Gum Rosin from Mead King Robinson Co Ltd, PEG 4000 (Lanster Chemicals) and lower molecular weight materials such as PE wax (AC 617A NE 3569, Allied Chemicals) are also effective.
The terms xe2x80x9cmisciblexe2x80x9d and more especially xe2x80x9ccompatiblexe2x80x9d should not be understood in any narrow sense of being free of all tendency (in The absence of other forces) to separate or segregate. Thus ionomers such as those sold under the Trade Mark xe2x80x9cSurlynxe2x80x9d are not considered miscible with LLDPEs, nor are they promoted as being compatible with LLDPEs. However we have shown that under the high stress mixing and shearing forces experienced in a screw extruder they can be finely and homogeneously dispersed to levels of say 10% w/w and any inherent tendency to segregate or for droplets to coalesce into large globules does not adversely manifest itself in the short duration of extrusion prior to consolidation of the tube.
The polyethylene-miscible or compatible resin which imparts melt strength to the polymer blend can be, for example, ethylene/vinyl acetate copolymer such as CIL 605-V or ethylene/methyl acrylate or ethylene/butyl acrylate (EMA or EBA esters) or a low density polyethylene having a melt index of 3 or less. Lupolen 2910 M is a suitable EBA ester obtainable from BASF (UK) Ltd.
Of course these polymers may include typical additives such as flame retardants antioxidants fillers, slip and antiblocking agents, coupling agents, U.V. stabilizers, thickeners and pigments as required.